For over half a century the Anglo-American "special relationship" has been a dominant force in world affairs. Today it is the engine of the global war on terror, and its enduring strength continues to confound and even infuriate leaders in continental Europe. Britain is the only nation the U.S. truly trusts as an ally; it is the British prime minister and not the German chancellor, the French president or the U.N. secretary-general, to whom the U.S. president looks first for partnership in addressing the big international security matters of the day.

Most Americans automatically think of Winston Churchill and Margaret Thatcher when conversation turns to the Anglo-American alliance, and Britain's Conservative Party is traditionally viewed in Washington as the U.K. home of the special relationship. Tony Blair, a steadfast support for America since 9/11, is acknowledged as an exception to the rule, a pro-American leader of a traditionally America-skeptic Labour Party.

There are, however, growing doubts among American policy makers that the resurgent Conservative Party led by David Cameron remains fully committed to the special relationship. There is now discussion in the White House, State Department and Pentagon of the possibility of dealing with a future Conservative-led British government that seeks to distance itself from the U.S. But the issue is also of concern on Capitol Hill -- among Republicans and Democrats. The notion that the special relationship could be heading for the rocks is no longer a fantasy, but a distinct political reality.

Shadow Foreign Secretary William Hague's Jan. 31 speech to the Royal Institute of International Affairs -- Chatham House -- spoke of a "solid but not slavish" alliance, and called for "the effective management of the relationship with the United States of America." These controversial words caused significant political damage here. It echoed Mr. Cameron's "liberal conservative" speech given on the fifth anniversary of 9/11, where he spoke of the need for "humility and patience" in conducting foreign policy.

Mr. Hague's carefully chosen description of America's and Britain's "loss of moral authority," and the need for Britain to shift more political weight to "the relationships of the Asia-Pacific region," is viewed in Washington as a fundamental reassessment of the special relationship, with far-reaching implications for both the U.S. and the U.K.

What was missing from the speech was any display of solidarity with the U.S. when thousands of British soldiers are fighting alongside their American counterparts in Iraq and Afghanistan, a stunning omission at a time of war. British forces have played an extraordinarily successful role in maintaining security in the southern third of Iraq in the face of Iranian interference and extreme provocation from Shia militias. In Afghanistan, British forces have been in the vanguard of NATO operations to defeat the Taliban and al Qaeda in the south of the country.

Perhaps the greatest benefit that Britain derives from the special relationship, and Tony Blair's close tie to George W. Bush, is the almost invisible security embrace, and close intelligence cooperation, that Washington gives only to its closest ally. U.S. intelligence, after all, helped thwart a series of large-scale al Qaeda attacks on British targets, including Heathrow Airport and Canary Wharf, which had been planned by 9/11 architect Khalid Sheikh Mohammed. Standing shoulder to shoulder with the U.S. may not be the most popular policy in Britain, with few votes to be gained, but it is fundamentally in Britain's interest.

The special relationship is a two-way street that significantly enhances Britain's ability to act as a major player on the world stage. For London, any downgrading of the Anglo-American alliance would significantly harm British strategic interests, and result in the loosening of defense and intelligence ties, the further loss of national sovereignty within the European Union, the diminution of British global power, and a weakening of the two nations' close-knit financial, trade and investment relationships.

Unfortunately, in its public statements the Conservative Party's leadership appears increasingly to be following the rise of anti-Americanism in the polls, rather than leading public opinion. There is a very real danger the Conservatives will make the same mistake that Gerhard Schröder did in the 2002 German elections, exploiting widespread anti-U.S. sentiment for short-term political gain. The consequences for U.S.-German relations were disastrous, with a virtual "cold war" between Washington and Berlin until Mr. Schröder's exit in 2005.

If the Conservatives take office in 2009 or 2010, the relationship between the White House and Downing Street could well be a tense one whether the Republicans or Democrats are in power, with a great deal of bad blood generated in advance. Despite President Bush's departure in January 2009, the fundamentals of U.S. foreign policy and America's approach to winning the war on terror are unlikely to dramatically change. Both sides of the political spectrum in Washington would balk at the kind of negative language being used by British Conservative politicians with regard to the Anglo-American alliance.

It is not too late, though, for the Conservatives to adopt a more traditional and pragmatic foreign-policy position, one that recognizes the huge stakes involved in maintaining the alliance. British and American leaders should advance a strongly pro-Atlanticist agenda that emphasizes Anglo-American global leadership.

The U.S. and Britain are committed to many of the same values and ideals on the world stage: the defense of national sovereignty, the projection of military power to confront tyranny and threats to international security, the advancement of free trade, and the protection of human rights.

With the threat of a nuclear-armed Iran just two to three years away, and the continuing war against al Qaeda raging across the globe, it is vital that Britain and America stand together. The future of the free world depends on it.

Kim R. Holmes is vice president for foreign policy and Nile Gardiner is director of the Margaret Thatcher Center for Freedom.